Electron impact spectrometer of high sensitivity and large helium tolerance and process of characterizing gaseous atoms and molecules by the energy loss spectrum

ABSTRACT

Process of and apparatus for measuring the energy loss of electrons in collisions with gaseous material, e.g., gaseous sample atoms and molecules for the purpose of characterizing (identifying, discriminating between or determining the molecular structure of) the atoms or molecules by the electron energy loss spectrum are provided. A current of low energy electrons is introduced into a region defined by being enclosed in a fine wire grid. The sample atoms or molecules are also in this region. The sample may or may not be diluted with helium gas. Electrons that have collided with the sample or with helium attempt to diffuse from this region out toward a collector electrode whose potential may be varied during the course of obtaining a spectrum. In general, if the kinetic energy of an electron is sufficient for it to surmount the potential hill (in the gravitational plot sense) or electrostatic potential between the collector and the grid, it will eventually be absorbed by the collector, since the area of the collector is purposely made enormously greater than that of the grid. If the kinetic energy of the electron is insufficient to reach the collector, i.e., is insufficient to surmount the potential hill or electrostatic potential between the potential of the grid and the potential of the collector, the electron is eventually absorbed on the grid. Thus the current to the grid is made up of those electrons that have lost sufficient kinetic energy in collisions with the sample a atoms or molecules that they cannot surmount the electrostatic potential between the grid and collector and reach the collector. The measure of this current as a function of the collector potential, e.g., the derivative of the current to the grid with respect to the collector potential, provides a spectrum of energy loss of the electrons particularly useful in characterizing, i.e., identifying, discriminating between, or determining the molecular structure, of the sample gaseous atoms or molecules.

llnite States Patent Ridgway Sept. 17, I974 [75] Inventor: Stuart L.Ridgway, Princeton, NJ.

[73] Assignee: Princeton Applied Research Corporation, West WindsorTownship, NJ.

221 Filed: Mar. 8, 1973 21 Appl.No.:339,544

[52] US. Cl. 250/305, 250/306 [51] Int. Cl. G0ln 23/02 [58] Field ofSearch 250/305, 306, 307

[5 6] References Cited UNITED STATES PATENTS 6/1972 Golden 250/305 OTHERPUBLICATIONS Electrical Differentiation For Energy Loss Analysis,Curtis, Journal of Physics E. (Great Britain) Vol. 3, Nov. 1970, 250305.

A New Method to Study Molecules By Electron Impact Spec, Knoop, ChemicalPhysics Letters, Vol. 5, No. 8, 6/70, 250-305.

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. ChurchAttorney, Agent, or Firm-Popper, Bain, Bobis, Gilfillan & Rhodes [57]ABSTRACT Process of and apparatus for measuring the energy loss ofelectrons in collisions with gaseous material, e.g., gaseous sampleatoms and molecules for the purpose of characterizing (identifying,discriminating between or determining the molecular structure of) theatoms or molecules by the electron energy loss spectrum are provided. Acurrent of low energy electrons is introduced into a region defined bybeing enclosed in a fine wire grid. The sample atoms or molecules arealso in this region. The sample may or may not be diluted with heliumgas. Electrons that have collided with the sample or with helium attemptto diffuse from this region out toward a collector electrode whosepotential may be varied during the course of obtaining a spectrum. Ingeneral, if the kinetic energy of an electron is sufficient for it tosurmount the potential hill (in the gravitational plot sense) orelectrostatic potential between the collector and the grid, it willeventually be absorbed by the collector, since the area of the collectoris purposely made enormously greater than that of the grid. If thekinetic energy of the electron is insufficient to reach the collector,i.e., is insufficient to surmount the potential hill or electrostaticpotential between the potential of the grid and the potential of thecollector, the electron is eventually absorbed on the grid. Thus thecurrent to the grid is made up of those electrons that have lostsufficient kinetic energy in collisions with the sample a atoms ormolecules that they cannot surmount the electrostatic potential betweenthe grid and collector and reach the collector. The measure of thiscurrent as a function of the collector potential, e.g., the derivativeof the current to the grid with respect to the collector potential,provides a spectrum of energy loss of the electrons partic-' ularlyuseful in characterizing, i..e., identifying, discriminating between, ordetermining the molecular structure, of the sample gaseous atoms ormolecules.

55 Claims, 6 Drawing Figures i I I I 1 I 4 r ANALYSIS SPACE I ELECTRODE& 25

- l0 SHIELDASAMPLE 1 ,17 HEATER PI ELECTRON SOURCE a EELECTRON LENSSYSTEM 0 s3 INJEcToRQ I ELECTROD pi, 92 F," asPAcE GRID ZCOSLPLAEEMQ I 9COLLECTOR S P E R SYSTEM IB INSULATION ,NACUU M ENCLOSURE l3 HELIUM GASCHROMATOGRAPH PATENIEU SEP 1 1974 sum 3 [If 5 A w. GEO

wmddm OP xUOJm PATENIEDSEFI mm SHEET Q 0F 5 FIGA ELECTRON IMPACTSPECTROMETER OF HIGH SENSITIVITY AND LARGE HELIUM TOLERANCE AND PROCESSOF CHARACTERIZING GASEOUS ATOMS AND MOLECULES BY THE ENERGY LOSSSPECTRUM BACKGROUND Prior art electron impact spectrometers for thepurpose of measuring the energy loss spectrum in collisions between anelectron beam and atoms and molecules have generally used the principleof preparing a moderately or highly monoenergetic beam of electrons in ahigh vacuum electron monochromator, then impacting them on a gas targetin a suitable differentially pumped target box, and then selecting some,or a range of angles from those electrons emerging from the target box,with a specific energy. The energy acceptance of the emergent electronanalyzing device is scanned to provide the energy loss spectrum.Although high spectral energy resolution is available in this manner,the current of analyzed electrons is small, and a long time is requiredto acquire a spectrum. Furthermore, the presence of an inert gas such ashelium in the sample causes a strong scattering of the electrons out ofthe beam, and further weakens the signal available. This reduces thepotential utility of the device for qualitative chemical analysis of themolecules that would emerge from a gas chromatograph, where dilutionwith helium is a most common feature of such samples.

SUMMARY The present invention provides a novel process forcharacterizing gaseous material, gaseous atoms and molecules, and anovel electron impact spectrometer, and which novel process andapparatus are particularly useful with samples that are carried in alarge excess of helium. Further, such novel process and apparatusprovide sufficient sensitivity so as to enable the acquisition of aspectrum in a fraction of a minute and with sufficient resolution toenable the spectra obtainable to be sufficiently characteristic of theparticular sample molecule or atom so that discrimination can be madebetween quite similar atoms and molecules.

DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic representation of aspectrometer system embodying the present invention and showing thoseelements that prepare the electron current, provide for its interactionwith the sample, and for the collection of the electrons after theirinteractions;

FIG. 2 is a schematic representation of a signal processing blockshowing, for example, those elements that apply the appropriate voltagesto the various electrodes and structures of the present invention;

FIG. 3 is a diagrammatic representation of a gas chromatograph inconnection with which the present invention is particularly useful;

FIG. 4 is a diagrammatic representation of means, according to thepresent invention, for providing energy selection on the electrons to beimpacted on the sample atoms or molecules to be characterized, and

FIGS. 5 and 6 are shown diagrammatic representations of space grids.

DESCRIPTION OF THE INVENTION In the description of the presentinvention, it will be understood by those skilled in the art that themotion of the electrons is considered to be in substantially constantelectrostatic fields, i.e., the fields are varied extremely slowlyrelative to the transit time of the electrons through the apparatus ofthe present invention, or the time of propagation of a light signalacross the apparatus. Furthermore, there are no substantial time varyingmagnetic fields. In this situation the electric field possesses apotential, whose gradient is the electric field. The normal unit ofpotential difference is the volt, and this will be used. The potentialenergy of a change q at a place where the electrostatic potential is qSis qzb. A field for which the potential energy function exists is calleda conservative field, and is a field for which the conservative ofenergy holds true as the particle moves through the field. In thisapparatus, therefore, as long as the electron moves in the electrostaticfields, and does not undergo inelastic collisions with molecules orsurfaces, the total energy is a constant of the motion where the totalenergy is defined by the well known relation E=T V, where E is the totalenergy, T the kinetic energy 1/2 mv and V the potential energy qdJ. Thecommon simplification that is frequently used in electron optics will beadopted by choosing as l the unit of energy the electron volt which isthe energy change in displacing one electron charge through a potentialdifference of one volt. Kinetic energy will be measured in electronvolts, and the energies of the ex cited states of molecules will be alsoso measured. Since the charge on the electron is negative, the electronpotential energy and the conventional electrostatic potential have, bythe above conventions, equal magnitudes and opposite signs. Thus it willbe said that the potential of a region is X volts, whereas the potentialenergy of an electron in such a region would be X electron volts. Thechoice of the zero of the electrostatic potential is arbitrary, and theonly physical measurable quantities are potential differences. It isponderous and confusing to continuously refer to potential differencesrelative to some origin, so therefore in this specification and claimswhen the terms potential or potential energy are used without referenceto a difference it will be understood that these potentials are referredto the reference zero of potential energy such that the average electronemitted from the electron source would have zero kinetic (and totalenergy) at this reference zero of potential. The thermionic cathode isapproximately at this so defined zero of potential, and it would beexactly so if the electrons emitted from it were emitted at zerovelocity. The small thermal velocity at which the electrons are actuallyemitted correspond to an energy of about /2 electron volt, so thecathode potential is thus /2 volt positive relative to the previouslydefined zero of potential, and the electrons leaving the cathode have /zelectron volt of potential energy, and /z electron volt of kineticenergy. By this definition of the zero of potential the electron totalenergy is zero after emission from the cathode, and remains so duringthe conservative motion through the apparatus. When it loses energy inan inelastic collision with a molecule or atom the totaI energy becomesnegative. The kinetic energy can never be less than zero. In all regionsaccessible to the original electrons the electrostatic potential ispositive, and the electron po tential energy is negative. Once anelectron has lost some energy in a collision, e.g., 6 electron volts,the total energy of this electron would be 6 electron volts. Only thoseportions of the apparatus that have an electrostatic potential morepositive than 6 volts, and a corresponding potential energy forelectrons more negative than 6 electron volts, are accessible to such anenergy degraded electron.

Referringnow to FIG. 1 and the spectrometer system shown therein, thereis shown an electron source 1 at a predetermined potential, the electronsource may be for example, a resistance heated tungsten or similarfilament that emits thermionic electrons. These electrons are introducedinto a collision space 24 by the assistance of an electron lens system 2and an injector electrode 20. The potential relative to the potential ofthe electron source is established by a surrounding space grid 8 whichis at a predetermined potential with respect to the potential of theelectron source. Upon being introduced into the collision space 24, theelectrons are provided with kinetic energy substantially equal inelectron volts to the potential in volts of the space grid with respectto the potential of the electron source 1.

The space grid 8 is substantially surrounded or enclosed by a collectorelectrode 9 which may have metal structures as shown formed upon it toform pockets for the entrapment of electrons. The collector electrode isalso at a predetermined potential with respect to the potential of theelectron source and, as taught in detail infra, the relative potentialsof the space grid and collector electrode may be varied. The spacebetween the collision space and the collector is called the analysisspace 25. It is in this space that the electrons are classifiedaccording to whether their kinetic energy is sufficient that they maysurmount the electrostatic potential (sometimes referred to by thoseskilled in the art as a hill in the sense of a gravitational plot)between the potential of the collector 9 and the potential of the spacegrid 8 relative to the potential of the electron source 1. The collector9 and the space grid 8 are both enclosed in a nearly completely closedconducting structure 10 referred to as the shield and sample can, sinceit serves both the function of shielding the spectrometer active regionfrom exter al electric fields, and also suffices to contain the sample.A sample of gaseous material, e.g., gaseous atoms or molecules, isintroduced through the sample inlet tube 6, and escapes from the samplecan 10 by way of the electron entrance orifice 33 in the injectorelectrode 20. If the escape of a sample after analysis is notsufficiently fast through this orifice 33, the valve 26 may be opened byenergizing solenoid 30 which attracts iron piece 28 against theresistance of spring 29, the actuation being transmitted by rod 27. Onrelease of the energization of solenoid 30, the spring 29 recloses valve26.

The spectrometer system is evacuated by vacuum pump system 14. Pressuresin the shield and sample can 10 may range between 10 torr to .l torr.The spec trometer system is enclosed in a vacuum enclosure 13. Heatingmeans 17 enclosed by insulating means 18 is provided for convenience incleaning and outgassing the spectrometer system, and for running thespectrometer at an elevated temperature when it is desirable forminimizing the absorption of the sample molecules on the surfacesinterior to the shield and sample can 10. Pressures in the vacuumenclosure 13 should be sufficiently low to get good operation of theelectron source 1 and the electron lens 2, and should be better thanabout 10 torr, and are typically in the range of 10 torr to 2 X 10 torrdepending on the sample loading.

With regard to the general operation of the spectrometer system of FIG.1, it will be assumed that the potential of the electron source 1 iszero volts and that the potential of the space grid 8 relative to theelectron source 1 is a fixed or constant +20 volts, a relative voltagegreater in volts than the expected loss of kinetic energy in electronvolts to be experienced by the electrons in colliding with the samplegaseous atoms or molecules. Thus, the electrons from source 1 that havepassed through the electron lens system 2 and orifice 33 and which areinside grid 8, and which have suffered no energy loss in collision witha sample atom or molecule in the collision space 24, will be providedwith kinetic energy of +20 electron volts. It will also be assumed thatthe potential of the shield and sample can 10 is approximately 4 voltsrelative to the electron source 1, thus no electron from the collisionspace 24 will have sufficient kinetic energy to reach it. The bottom ofthe shield and sample can 10 may be provided with a slightly inclinedportion generally opposite the entrance of the electron beam to form anelectron mirror 12. Thus the electrons may make a multitude ofoscillations up and down through the collision space 24 being reflectedby the shield and sample can 10, and not escaping through the potentialvalley or pass provided by the injector electrode 20 for the entrance ofthe electron beam. Upon coming close to the space grid 8, or beingscattered by a sample gas atom or molecule, the path of the electron ischanged, and it may be directed toward the collector electrode 9. If thepotential of the collector electrode 9 is above (more positive) +20volts, all electrons make it to the collector electrode on the firsttry. As the potential of the collector electrode relative to thepotential of the electron source 1 is decreased (becomes less positive),the number of tries that an electron must make in order to find that itsmotion is sufficiently directed against the electrostatic potential orpotential barrier between the space grid 8 and the collector 9,increases; thus, it will be understood by those skilled in the art thatthe potential of the collector electrode is a retarding potential. Ingeneral, each time the electron reenters the collision space 24, it maybe scattered in a new direction to enable it to make it to the collectorelectrode on the next try. In the absence of collisions with sampleatoms or molecules to be characterized, close passes of the grid wirescomprising the space grid 8 serve the same function. It is important forgood energy resolution that the wires comprising the space grid 8 bevery fine, and openly spaced in order that most of the electrons thatare collected or adsorbed by the space grid are those that areenergetically incapable of making it to the collector 9; i.e., electronsthat have insufficient kinetic energy to surmount the electrostaticpotential or potential barrier between the space grid- 8 and collectorelectrode 9. It is also important for good energy resolution that thegrid be supported in a manner that the area of conducting material atthe potential of the space grid be not increased substantially over thatof the space grid, thus the metal that supports the grid in theembodiment shown in FIG. 6 is maintained at a negative potential; andthe grid in the embodiment shown in FIG. 5 is supported on high qualityinsulating material.

It has been found to be particularly advantageous in the achievement ofthe desired energy resolution in the practice of the present invention,to structure the space grid 8 such that the probability of the captureor collection of an electron by the space grid 8 in a single pass of theelectron from the analysis space 25 through the collision space 24 andon to the analysis space 25 again, be less than .03. The current signalprovided by collecting the electrons could be taken as the collectorelectrode current, or as the current to the space grid, or as thedifference of the two.

It is preferable, although not essential to take the signal from thespace grid. There are two advantages to taking the signal from the spacegrid. The first is that the shot noise of the base current is the majorsource of noise in the system, and the noise that limits the ultimatesensitivity of the system. In the spectral region where the interestinglines of many compounds lie, the current to the space grid is small, andthe current to the collector electrode is large. Since the shot noisepower is proportional to the current, the signal is available with lessaccompanying shot noise at the space grid. The second advantage of usingthe space grid as the signal output electrode is that the capacity ofthis electrode to ground, and to other electrodes is very small. Thisminimizes the contribution of the voltage noise of the amplifier to theoutput noise level. Accordingly, as shown in FIG. 2, the space grid isconnected via lead 16 to the input of low noise amplifier 35. A feedbackresistor R, is connected from the output of this amplifier to the inputin the well known virtual ground configuration. The amplifier isfloated, the other terminal being connected to the voltage source thatis set or controlled to the potential to which it is desired to set thespace grid 8.

It will be further understood'by those skilled in the art that if thesample characterizing current is provided by electrons collected at thecollector electrode 9, such current will be comprised of electronshaving sufficient kinetic energy to surmount the electrostatic potentialbarrier, between the space grid 8 and collector electrode 9, includingelectrons having sufficient residual energy after colliding with thegaseous sample atoms or molecules. Further, if the sample characterizingcurrent is provided by electrons collected at the space grid 8, suchcurrent will be provided by electrons having insufficient residualkinetic energy after colliding with the sample gaseous atoms ormolecules to surmount the electrostatic potential barrier between thespace grid and the collector electrode upon the difference in potentialin volts between the collector electrode and the electron sourcepotential being less than the kinetic energy in electron volts lost bythe electrons in collisions with the sample.

To further understand the operation, further reference is made to FIG. 2commencing with a suitable programmer 46. This programmer may becoupled, for example, to the peak detector of a gas chromatograph. Onceit is apparent that there is sufficient material from the chromatographto enable a spectrum to be run, and the sample flow time from thethermal conductivity detector and the charging time of the spectrometervolume have been properly allowed for, a signal is sent to a suitablesweep source 37. The potential outputs of the sweep source arereferenced to the electron source via lead 23 to the spectrometer. Thesweep voltage is transmitted to a suitable display device, such as asuitable x-y plotter 39, and to further signal processing devices suchas a suitable computer 42 via a multiplexer 40 and a suitable analog todigital converter 41. The sweep is also sent to the excitation control44.

One mode of operating the spectrometer is to keep the potential of thespace grid 8 fixed or constant while the collector electrode voltage isbeing swept or varied to scan the spectrum; for example, and inaccordance I with the present invention, the potential of the collectorelectrode relative to the electron source potential is swept between avalue more positive than the potenial of the space grid relative to theelectron source potential, to a value negative with respect to theelectron source potential. It has also been found advantageous to sweepor vary the voltage of the space grid 8 as the voltage of the collectorelectrode 9 is swept or varied to scan the spectrum. One mode isproportional varying or sweeping in that the space grid potentialrelative to the electron source potential is made some constant factortimes the collector electrode potential relative to the electron sourcepotential, this factor usually being between l.l and 2.0. The advantageof sweeping or varying the space grid potential is that in general forwide classes of compounds the lower energy lines of the molecules arebetter excited with electrons of lower energy than is necessary toexcite higher energy levels. Another mode of excitation of considerableutility is to sweep the potential of the space grid 8 in parallel withthat of the collector electrode 9, with the potential .of the space gridbeing maintained at a predetermined'constant difference of at least onevolt, typically between l and 6 volts, above (more positive) that of thecollector electrode. In this mode, the resolution of the energy analyzeris constant, since one is always looking at those electrons that haveresidual kinetic energy after the energy loss collision with sampleatoms or electrons equal to the difference in potential between thespace grid and the collector potential. It will be understood by thoseskilled in the art that such relative varying of the potential of thespace grid 8 also varies the kinetic energy provided to the electronsaccordingly.

The gaseous sample is characterized by the current of collectedelectrons, in the various alternative modes taught above, and moresignificantly in accordance with the present invention, the gaseousmaterial is characterized by being measured as a. function of theelectrostatic potential established by difference in potential betweenthe respective potentials of the space grid 8 and collector electrode 9with respect to the potential of the electron source. Still moreparticularly, the gaseous material may be characterized by measuring thecurrent as a function of the electrostatic potential by taking thederivative of such current with respect to the potential of the spacegrid 8 or the collector electrode 9 with respect to the potential of theelectron source 1.

In one embodiment of the invention, and still referring to FIG. 2, thederivative of the space grid current with respect to the varyingcollector electrode potential was taken to characterize the samplegaseous material and obtained by an ac signal. technique. A suitableoscillator 43 operating at a frequency f provides a small ac modulatingvoltage of approximately 50 mV rms which is added to the sweep voltagesupplied to the collector electrode 9. To understand this method ofmodulation, it will be understood that at a particular collector tospace grid potential difference, electrons of greater energy than thispotential difference will collect on the collector electrode with highprobability, while electrons with less energy than this potentialdifference are compelled to be collected on the space grid. If there isa transition in the sample atoms or molecules such that there is a classof electrons with energy equal to the potential difference between thespace grid electrode and the collector electrode (reference value), thisclass of electrons will be partitioned between the space grid andcollector electrodes. If. however. the potential difference between thespace grid and the collector electrode is slightly lowered, theelectrons, after the energy loss collision, have energy which will beslightly greater than this potential difference and most of theelectrons of this collision class will make it to the collectorelectrode. Similarly, let it be assumed that the potential differencebetween the space grid and the collector electrode be raised above theoriginal reference value, i.e. above the difference in potential betweenthe space grid and collector electrode. Under this condition theelectrons cannot reach the collector electrode, and will be collected bythe space grid electrode. Thus, it will be understood that upon themodulation of the space grid collector electrode potential difference,there will be an alternating current to the space grid equal to theproduct of the slope of the space grid vs. collector voltage functionand the amplitude of the space grid-collector electrode potentialdifference modulation.

Thus, it will be understood that the ac modulation voltage provided bythe oscillator 43 applied to the sweep voltage applied to the collectorelectrode 9 alternately raises and lowers the space grid-collectorpotential difference above and below the above-noted reference value.Upon such ac modulation, the electron provided current to the space grid8 will have an ac component substantially equal to the product of themodulation amplitude applied to the collector electrode and thederivative of the space grid current with respect to the collectorvoltage. This is the desired electron energy loss spectrum. The spacegrid current signal is converted to a voltage signal by low noiseamplifier 33. This output signal is transmitted to amplifier 34, whichfurther amplifies the signal, and removes from the output of amplifier33 that component of the output that is due to the sweeping of thepotential of the space grid 8. If collector voltage modulation is used,then the output signal from amplifier 34 may be processed in suitablelock-in amplifiers 34 and 45. Lock-in amplifier 35 has a reference forits mixer that is the modulating signal developed by the oscillator 43.Further sharpening and enhancement of detail in the spectra may beobtained by the use of further lock-in amplifiers whose mixer referenceis a 2nd, 3rd, or higher harmonic in coherent phase relation with thefundamental modulating signal applied to the collector. The use of thehigher harmonics gives spectra that are a higher derivative of the spacegrid current versus collector voltage function. A lock-in amplifier 45operating on the third harmonic reference is shown. Depending upon thetype of display device available, the outputs of the two lock-ins may bedisplayed alternatively or simultaneously.

Alternatively, the output signal from amplifier 34 may be amplified inamplifier 36, and the appropriate derivatives of the space grid currentwith respect to the collector electrode potential may be obtained bymeans of filter 38, and the signal transmitted to the display device 39.Alternatively, the signal from amplifier 34, and the sweep from thesweep source 37 may be conducted to the digital computer 42 by way ofthe time division multiplexor 40 and the analog to digital convertor 41.The digital computer may process the spectral information, display thespectra in analog devices, match them with other spectra stored inmemory, and provide signals to control the programmer and the excitationcontrol, and other components of the system.

As noted above, although a gas chromatograph is not necessary as asample source for the system, the present invention has utility separateand apart from chromatography, one of the major advantages of thepresent invention is that it is uniquely able to process samples thatare obtained from a gas chromatograph. One method of coupling thepresent invention to a gas chromatograph is illustrated in the blockdiagram of FIG. 3. The sample to be characterized, which sample ingeneral will be a mixture of materials that it is desired to separate bythe chromatograph column, is injected into the column at the sampleinlet 60. The flow of helium from the helium cylinder 20 sweeps thesample through the column, and sweeps different materials with differentaffinities for the material of the column at different rates. Thecomponents emerge from the column separated in time carried by thehelium gas. The presence of a component in the carrier gas is detectedby a suitable thermal conductivity detector 47. ln order to enhance thesensitivity for components present in low concentration, some of thehelium may be removed from the stream by a suitable separator 48 such ascommonly used for the same purpose with mass spectrometer peakidentifiers. The discarded helium is removed by a suitable mechanicalvacuum pump 49. Between the output of the separator 48 and thespectrometer inlet 6 may be provided suitable reservoirs 50, 51 and 52for holding the various components as they emerge from the column andseparator, and for scheduling them into the spectrometer inlet 6 after aprevious peak has been analyzed. These reservoirs can serve anadditional function of providing a reserve of material so that theconcentration of analyte in the spectrometer chamber chamber 10 may bemaintained constant during the analysis of a sample.

With regard to the general operation of a cycle of the chromatograph ofFIG. 3, it will be assumed that the shield and sample can has beenemptied through valve 26, that the reservoirs are empty by having beenevacuated through valves 61, 62 and 63, and that the output of theseparator 48 has been going to waste through valve 59. On the appearanceof a signal from the terminal conductivity conductor 47, programmer 46(F162) allows for the transit through the separator and opens valve 53to admit sample to reservoir 50, and a valve 61 is closed. When asubstantial amount of the peak has passed into reservoir 50, valve 56 isopened full briefly to allow the transfer of about half the material tothe spectrometer shield and sample can 10 (FIG. 1), and then closed to asteady flow rate to maintain the concentration at a steady value. As thepeak passes the thermal conductivity detector 47 the valve 53 is closedand valve 59 opened to send the helium to waste. As a new peak arrivesas sensed by the thermal conductivity detector 47, valve 62 is closed,valve 59 is closed and valve 54 is opened to admit the material in thenext peak to the reservoir 51. Upon completion of the analysis of thematerial in reservoir 50, and the charging of the reservoir 51 with thematerial in the second peak from the gas chromatograph, valve 57 iscycled as was previously cycled valve 56 to admit the sample to thespectrometer shield and sample can 10 (FIG. 1), and the analysis of thissample is commenced. As new peaks come off of the gas chromatograph, thereservoirs 50, 51 and 52, and their valves are operated to service thepeaks. Three reservoirs have been chosen for illustration, it will beunderstood that more or fewer can be chosen depending upon the amount'ofclose bunching of peaks to be anticipated, which close bunching wouldrequire a sample queue of greater holding capacity.

For increased resolution it becomes highly desirable and advantageous toprovide energy selection means on the electrons emitted from theelectron source 1 of FIG. 1 so as to provide a stream of electrons tothe collision space 24 having a predetermined kinetic energydistribution about a predetermined energy mean or average. For example,if the electron source 1 is a heated tungsten filament operating at asuitable temperature, such as 2600Kelvin, the width of the energydistribution of the thermionic electrons is of the order of .5 volts,aand limits the attainable resolution of the present invention. It isalso not necessary to provide the shield and sample can 10 in that thecollector electrode 9 can be structured to provide for the'containmentof the sample to be characterized. These alternatives, in accordancewith the present invention, are illustrated in FIG. 4. The electronpre-analysis means or energy selection means 63 may include acylindrical electrostatic energy analyzer, comprised of cylindricalattractor electrode 70, and cylindrical repeller electrode 69, mountedby insulating means (not shown) upon a suitable support or frame 64,which frame supports, or contains spectrometer entrance slit 65, andspectrometer exit slit 66. Electrons from an electron source, e.g.,electron source 1 of FIG. 1, either directly, or through an electronimaging system (not shown), enter the spectrometer at the entrance slit65. Electrons of the correct and selected energy transverse a circularorbit, and exit the spectrometer via the exit slit. Electrons of higherthan the selected energy traverse the analysis region between the plates69 and 70 with a larger radius of curvature, and do not exit via theexit slit. Some ap preciable fraction of the higher energy electronswill be collected upon beam steering signal electrode 67. Electrons oflesser energy and thus radius of curvature will fall short of the exitslit, and a substantial fraction of them will be collected by beamsteering signal electrode 68. The length of the arc of the electronorbit is chosen to be approximately 127 to obtain the well known singlefocusing of cylindrical electron energy analyzers. At the price of asmall increase in cost and complexity, the well known double focusingschemes using 180 deflection in an inverse square law electrostaticfield using concentric hemispheres could be used. Typical operatingparameters for the cylindrical electron energy analyzer with equilibriumradius of curvature 2.4 inches, repeller electrode inner radius ofcurvature 2.6 inches, and attractor electrode outer radius of curvature2.2 inches were electron source potential 0.0 volts, attractor potential+l5.0 volts, repeller potential +l0.7 volts, and frame potential +l3.5volts. The frame potential sets the potential of the entrance aand exitslits, and whether the electrons are retarded or accelerated as theytraverse the region from the slit to the analysis region between theelectrodes 69'and 70. Deviation of the frame potential from a valueexactly intermediate to the attractor potential allows minor adjustmentsto be made in the focusing properties of the spectrometer, and thespectrometer may be tuned for increased intensity. Pre-analyzed electronstreams to be provided to the collision space 24, in accordance with theteaching of the present invention and as may be provided by the electronpre-analysis means 63, may advantageously be provided with a kineticenergy distribution of approximately 100 mV full width at half maximum.Higher resolutions are available at a lesser current.

The potentials on the electrodes will be chosen such that electrons of13 volts kinetic energy exactly traverse the equilibrium orbit, betweenthe attractor and repeller electrodes, of 2.4 inches radius ofcurvature. In order to provide such electrons the electron source mustbe different in potential from the electrostatic potential of theequilibrium orbit by 13 volts less the average kinetic energy of thermalemission of the electrons, which may be a few tenths of a volt. Ascathode conditions change, due to source aging, absorption or desorptionof sample material or other gas in the vacuum system, or temperaturechange, the energy acceptance of the spectrometer may not be centeredupon the electron energy distribution of the electron source. This willcause a loss of intensity of the beam, and a small shift in the averageenergy of the analyzed electrons, and a consequent change in theposition of the spectral features observed by the instrument. Theseeffects can be adequately allowed for in the method of spectra analysis,in that the collector voltage be scanned across the energy of thenoninteracted component of the electron beam to measure the initial beamenergy. Then the positions of the spectral features may be measured withrespect to this zero energy loss peak. However, it is a greatconvenience to have automatic means for keeping the position of thiszero energy loss peak fixed, and to keep the beam intensity maximized.If the average energy of the electrons provided by the electron sourceis greater than that for which the energy analyzer is set, then thecurrent to electrode 67 is enlarged, and that to electrode 68 isdiminished. Thus the current signals from these electrodes are conductedto differential amplifier 91 shown in FIG. 5, electronic service blockvia leads 82 and 83. The output of this amplifier, which is a smallnegative or positive voltage depending upon whether the current toelectrode 68 or to electrode 67 is larger, is added to the electronsource potential pro vide via lead 15.

The introduction of the electron energy analyzer of FIG. 4 makes analternative method of modulation attractive.

The alternative method of modulation (i.e., alternative to theabove-mentioned modulation of the spacegrid collector electrodepotential difference) is to apply the ac modulating voltage developed byoscillator 43 (FIG. 2) to the potential difference between the attractorand the repeller 69 and 70 electrodes of FIG. 4. To understand thismethod of modulation, it will be understood that at a particularcollector to space grid potential difference, electrons of greaterenergy than this potential difference will collect on the collectorelectrode with high probability, while electrons with less energy thanthis potential difference are compelled to be collected on the spacegrid. If there is a transition in the sample atoms or molecules suchthat there is a class of electrons with energy (reference value) equalto the potential difference between the space grid electrode and thecollector electrode, this class of electrons will be partitioned betweenthe space grid and collector electrodes. If, however, the energy of theincident electron beam or stream exiting the pre-analyzer 63 is slightlyraised above the reference value, the electrons, after the energy losscollision, will have their energy also slightly raised by the sameamount, because of the discreteness of the energy acceptances by thetarget molecules; most of the electrons of this collision class willmake it to the collector electrode.

Similarly, let it be assumed that the energy of the incident electronstream of beam be lowered below the original reference value. Under thiscondition the electrons cannot reach the collector electrode, and willbe collected by the space grid electrode. Thus, it will be understoodthat upon the modulation of the energy of the incident electron beam,there will be an alternating current to the space grid equal to theproduct of the slope of the space grid vs. collector electrode voltagefunction and the amplitude of the electron energy modulation. Theadvantage of this method is that the stray capacitances between thespace grid and the energy analyzing electrodes can be made very small,so that there is very little background current in the signal channel.It has been found that the amount of ac modulation may be chosenadvantageously such that the amount of modulation in the energy of theelectron beam exiting from the electron energy pre-analyzer 63 isapproximately between 50 and 150 millivolts.

The ac components of the electron provided current to the space grid issubstantially equal to the product of the amplitude of the energymodulation of the electron beam consequent to the modulation applied atthe pre-analyzer repeller and attractor electrodes, and the derivativeof the space grid current with respect to the collector voltage. Thisalso provides the desired energy loss spectrum.

It will be still further understood by those skilled in the art that thechanges in current to the space grid and to the collector electrode aretransfers of current from one electrode to another, and that the changesin each current are of opposite sign at the two electrodes. Thesecurrents may be conducted to a differential amplifier to gain a largersignal, and a reduction of certain types of noise that might be ridingon the total electron beam. In the case of electron beam, energymodulation taught above, the output of the differential amplifier willbe proportional to the algebraic sum of the ac currents to eachelectrode individually, which individual currents depend upon slopes andmodulation as described above.

For the best functioning of the electron energy analyzer of FIG. 4 inthe ordinary magnetic field of a typical laboratory a magnetic shield-80is provided. It is a cylinder closed at one end, and extendingapproximately one diameter or more beyond the electron energy analyzerin the direction of the open end. A magnetic shielding material of highpermeability such as molybdenum permalloy should be used, and preferablyin the fully annealed condition. For a nine inch diameter cylinder athickness of .050 incheshas been found to be satisfactory.

It is important for the functioning of the spectrometer of the presentinvention as an instrument of good resolution and high sensitiwity, thatthe ratio of the area of the space grid electrode 8 for interception ofthe electrons diffusing through the collision be as small as possible.In FIG. 5 is shown one embodiment which realizes this requirement, andin FIG. 6 there is shown a second embodiment which also realizes thisrequirement. In FIG. 5 there is shown the space grid electrode woundupon an insulating framework substantially cubical in outline. Typicaldimensions which have been found to be satisfactory and which arepresented merely by way of example, would be for the cube to beassembled of 2 mm dia pyrex or Corning grade 7070 glass rods. The lengthof the cube edges is about 10 cm. Four of the rods, forming edges 91,92, 93 and 94 are extended to form insulating supPorts of the structureupon the collector or upon the shield and sample can. These supports areshown in FIG. 4 as being from the collector, and in FIG. 1 as being fromthe shield and sample can 10. The space grid electrode may be wound ofvery fine wire, e.g. .0003 inch diameter. In the form of the gridwinding shown in FIG. 5 the space grid wire 103 is tied to the frameedge 97, is carried up to frame edge 100, then over to frame edge 101,then down to edge 95, and then over to the beginning edge 97. Thewinding is continued in this manner with a spacing between turns ofapproximately .2 inches, more or less. When the four faces have beencovered by the winding, the last turn is brought up from frame edge 97to frame edge 100 in the neighborhood of its intersection with frameedge 94, the wire is then brought down over frame edge 102 to frame edge98, then over to frame edge 96, up to frame edge 99 and back over toframe edge 102. The winding is continued in this manner until the twofaces that were uncovered are covered. In this process the top andbottom faces are covered a second time with wire that is approximatelyperpendicular in direction to the first winding over these faces. InFIG. 5 the winding is interrupted at position 104 for improved clarity.The last partial turn of this winding is shown starting at position 105,carried over frame edge 96, and up the intersection of frame edge 92 andframe edge 92 and frame edge 99. Here it is tied to frame edge 92, whereit is then carried up through a penetration in the sample can that hassmall gas conductance and good insulating value to become space gridsignal lead 16. When the winding is brought from frame edge 100 to frameedge 102, it may be tied to frame edge 94 for additional security. Thewire that is best for the space grid is too fine for convenience as asignal lead, and it has been found convenient to splice the grid wire103 to a larger diameter lead wire 16 to ease the bringing out of thelead through the container that holds the gas sample. This structureprovides a large volume for interaction of electrons and the sample gasand the interior of this structure is mostly at a relatively constantelectrostatic potential, namely that of the space grid itself.

In FIG. 6 there is shown an alternative embodiment for providing thespace grid electrode. In this embodiment the wire forming the space gridelectrode is wound in a single spiral over the four insulating rods 110,111, 112 and 113. As before, these rods may be 2 mm dia pyrex or Corninggrade 7070 glass. The rods are supported by a rectangular sheet metalframe composed of end pieces 106 and 107, and side pieces 108 and 109the frame is made of suitable electrically conductive metal, e.g.,copper. End piece 107 has a hole 117 in it to allow the entrance of theelectron beam along axis 114. Clearance for insulating purposes isprovided between the winding and the side pieces 108 and 109. Thewinding is shown starting at 103, brought over to insulating rod 113,and up to insulating rod 112. The winding is continued in this manneruntil the faces are covered, and then tied to the insulating rod 112,and then continued as space grid connection lead 16. The metal frame ismaintained at a potential below that of the electron source 1, so thatit cannot accept significant electron current. The potentialdistribution in the .box formed of the frame, and the grid wound overits open faces has a maximum in the center. The electron beam isintroduced off the center of this box. The potential distributionprovides radial electrical forces that tend to cause an orbiting of theinjected beam around the geometrical center of the box. This arrangementprovides a high probability for interaction of the beam with the samplegas molecules or atoms, and a simple method of winding the space gridelectrode.

What is claimed is:

1. The process of characterizing gaseous material, comprising the stepsof:

surrounding said gaseous material in a region of predetermined potentialwith a predetermined electrostatic potential barrier;

impacting said gaseous material with electrons of predetermined totalenergy;

collecting electrons having a predetermined relationship between theirkinetic energy and the potential of said predetermined their kineticenergy and the potential of said predetermined electrostatic potentialbarrier to provide a current, said current characterizing said gaseousmaterial; and measuring said current.

2. The process according to claim 1 wherein said collected electronsproviding said current are related to the potential of saidelectrostatic potential barrier by having insufficient residual kineticenergy after colliding with said gaseous material to surmount saidelectrostatic potential barrier.

3. The process according to claim .1 wherein said collected electronsproviding said current are related to the potential of saidelectrostatic potential barrier by having sufficient kinetic energy tosurmount said electrostatic potential barrier and wherein said currentincludes electrons having sufficient residual kinetic energy aftercolliding with said gaseous material to surmount said electrostaticpotential barrier.

4. The process according to claim 1 including the further step ofprocessing said current to characterize said gaseous material.

5. The process according to claim 4 including the further step ofmodulating one of the potentials which establishes said predeterminedelectrostatic potential barrier and wherein said processing comprisesthe step of measuring said current as a function of said modulatedpotential.

6. The process according to claim 5 wherein said processing comprisesthe step of taking the derivative of said current with respect to saidpotential which is modulated.

7. The process according to claim 1 wherein said electrostatic potentialbarrier is varied over a predetermined range.

8. The process according to claim 1 including the further step ofmodulating the energy of said electrons over a predetermined energyrange. i i i i 9. The process according to claim 1 wherein saidelectrostatic potential is provided by two potentials and wherein one ofsaid potentials is varied in a predetermined manner with respect to theother.

10. The process according to claim 9 wherein said one potential isvaried proportionally with respect to said other potential.

11. The process according to claim 9 wherein said one potential isvaried to provide a constant predetermined potential difference betweensaid one potential and said other potential.

12. The process according to claim 1 wherein said predeterminedpotential of said region is a substantially uniform potential.

13. The process according to claim 1 wherein said predeterminedpotential of said region is a varying potential.

14. The process according to claim 1 including the further step ofpre-analyzing the kinetic energy distribution of said electronsimpacting said gaseous material so as to impact said gaseous materialwith a stream of electrons having a predetermined energy distributionabout a predetermined energy average.

15. The process according to claim 14 wherein said energy distributionis approximately millivolts full width at half maximum.

16. The process according to claim 1 including the further step ofmodulating said varying retarding potential.

17. The process according to claim 16 wherein said energy distributionis approximately 100 millivolts full width at half maximum.

18. The process of characterizing gaseous material, comprising the stepsof:

providing a stream of electrons from an electron source at apredetermined potential;

providing a collision space at a predetermined potential relative tosaid potential of said electron source, said predetermined relativepotential being greater in volts than the kinetic energy lost by saidelectrons in electron volts upon said electrons colliding with saidgaseous material;

substantially surrounding said collision space with a variable retardingpotential at a predetermined potential realtive to said potential ofsaid electron source;

evacuating the region occupied by said collsion space and said variableretarding potential to a predetermined pressure;

introducing said gaseous material into said collision space;

introducing said electrons into :said collision space to provide saidelectrons with predetermined kinetic energy substantially equal inelectron volts to said predetermined potential of said collision spaceand impacting said gaseous material with said electrons provided withsaid predetermined kinetic energy;

varying the predetermined potential of said variable retarding potentialover a predetermined potential range;

providing a current by collecting electrons having insufficient residualkinetic energy after colliding with said gaseous material to surmountthe potential difference between the potential of said collision spacerelative to said electron source and the varying potential of saidvariable retarding potential relative to said electron source upon thepotential diffference between said variable retarding potential and saidpotential of said electron source being at least less in volts than thekinetic energy lost by said electrons in electron volts upon saidelectrons colliding with said gaseous material; and

measuring said current as a function of said varying retardingpotential.

19. The process according to claim 16 including the further step ofvarying the potential of said collision space relative to said potentialof said electron source to vary the kinetic energy provided saidelectrons over a predetermined kinetic energy range.

20. The process according to claim 19 wherein the potential of saidcollision space relative to said potential of said electron source isvaried in a predetermined manner with respect to said varying potentialof said surrounding retarding potential relative to said potential ofsaid electron source so as to vary said kinetic energy provided saidelectrons in a predetermined manner with respect to said varyingrelative potential of said surrounding retarding potential.

21. The process according to claim 20 wherein said potential of saidcollision space relative to said potential of said electron source isvaried proportionally with respect to said varying potential of saidsurrounding retarding potential with respect to said potential of saidelectron source so as to vary the kinetic energy provided said electronsproportionally with respect to said varying potential of saidsurrounding retarding potential with respect to said potential of saidelectron source.

22. The process according to claim 20 wherein said potential of saidcollision space relative to said potential of said electron source isvaried to provide a predetermined constant potential difference withrespect to said varying potential of said surrounding retardingpotential with respect to said potential of said electron source so asto provide said electrons with varying kinetic energy in electron voltswhich is at a predetermined constant difference with respect to saidvarying potential of said surrounding retarding potential with respectto said potential of said electron source in volts.

23. The process according to claim 22 wherein said predeterminedconstant potential difference is at least one volt.

24. The process according to claim 22 wherein said predeterminedconstant potential difference is between 1 and 6 volts.

25. The process according to claim 18 wherein the potential of saidcollision space relative to said potential of said electron source is apredetermined relative positive potential and wherein the potential ofsaid surrounding variable potential relative to said potential of saidelectron source is varied between a relative potential more positivethan said relative positive potential of said collision space and arelative potential which is negative with respect to said potential ofsaid electron source.

26. The process according to claim 18 wherein said step of measuringsaid current as a function of said varying retarding potential comprisesthe step of taking the derivative of said current with respect to saidvarying retarding potential.

27. The process according to claim 18 including the further step ofpre-analyzing the kinetic energy distribution of said electrons fromsaid electron source to provide a stream of electrons for impacting saidgaseous material having a predetermined energy distribution about apredetermined average.

28. The process according to claim 27 including the further step ofmodulating the energy of said stream of electrons.

29. The process of characterizing gaseous material, comprising the stepsof:

providing a source of electrons, each of said electrons from said sourcehaving approximately the same energy; providing a first region of spaceover which the potential energy of an electron relative to the referencezero of potential energy, which reference zero of potential energy isdefined as that potential energy for electrons at which the averageenergy electron from said electron source would possess zero kineticenergy, is substantially negative over a substantial portion thereof,said first region of space containing said gaseous material; providing asecond region of space surrounding said first region of space and overwhich second region of space the potential energy of electrons increaseswith separation from said first region of space;

bounding said second region of space with a surface at a predeterminedvariable potential relative to said reference zero of potential, uponsaid electrons from said source being introduced into said first regionof space and impacting said gaseous material, certain of said electronspassing through said surface and providing a current depending inmagnitude upon said predetermined variable potential, said dependencecharacterizing said gaseous material directing electrons from saidsource into said first region; and measuring said current as a functionof said variable potential.

30. Apparatus for characterizing gaseous material, comprising:

means forproviding a stream of electrons;

means for surrounding said gaseous material with an electrostaticpotential at a predetermined potential, for providing said electronswith predetermined kinetic energy and for impacting said gaseousmaterial with electrons provided with predetermined kinetic energy; and

means for measuring current provided by electrons having a predeterminedrelationship between their kinetic energy and said predeterminedpotential of said electrostatic potential, said current characterizingsaid gaseous material.

31. Apparatus according to claim 30 wherein said current measuring meansis for measuring current provided by electrons that are related to saidpredetermined potential of said electrostatic potential by havinginsufficient residual kinetic energy after colliding with said gaseousmaterial to surmount said electrostatic potential.

32. Apparatus according to claim 30 wherein said current measuring meansis for measuring current provided by electrons that are related to saidpredetermined potential of said electrostatic potential by havingsufficient kinetic energy to surmount said electrostatic potential andwherein said measured electrons include electrons having sufficientresidual kinetic energy after colliding with said gaseous material tosurmount said electrostatic potential.

33. Apparatus according to claim wherein said current measuring meanscomprises means for measuring said current as a function of saidpredetermined electrostatic potential.

34. Apparatus according to claim 30 wherein said current measuring meanscomprises means for taking the derivative of said current with respectto said predetermined electrostatic potential.

35. Apparatus according to claim 30 further including means for varyingsaid electrostatic potential over a predetermined range.

36. Apparatus according to claim further includ ing means for varyingsaid kinetic energy provided said electrons over a predetermined kineticenergy range.

37. Apparatus according to claim 36 wherein said means for measuringsaid electron current to said grid means measure said electron currentas a function of said varying potential of said electrode means, saidfunction characterizing said gaseous material.

38. Apparatus according to claim 36 wherein said means for providingsaid electrons with varying kinetic energy provides said electrons withkinetic energy which is varied in a predetermined manner with respect tosaid varying electrostatic potential.

39. Apparatus according to claim 38 wherein said means for varying saidkinetic energy provided said electrons varies said kinetic energy toprovide a constant predetermined potential difference between saidvarying kinetic energy and said varying electrostatic potential.

40. Apparatus according to claim 38 wherein said means for measuringsaid electron current as a function of said varying potential of saidelectrode means is for taking the derivative of said electron currentwith respect to said varying potential of said electrode means.

41. Apparatus according to claim 38 wherein said means for varying saidkinetic energy provided said electrons varies said kinetic energyproportionally with respect to said varying electrostatic potential.

42. Apparatus according to claim 41 wherein said derivative is thesecond derivative of said electron current with respect to said varyingelectrode means potential.

43. Apparatus according to claim 41 wherein said derivative is the thirdderivative of said electron current with respectto said varyingpotential of said electrode means.

44. Apparatus for characterizing gaseous material, comprising the stepsof:

means for providing a stream of electrons from an electron source at apredetermined potential;

means for providing a collision space at a predetermined substantiallyuniform, potential relative to said potential of said electron source,said predetermined relative potential being greater in volts than thekinetic energy lost by said electrons in electron volts upon saidelectrons colliding with said gaseous material, and said collision spacecontaining said gaseous material;

means for introducing said electrons into said collision space to impactsaid gaseous material;

means for substantially surrounding said collision space with a variableretarding potential at a predetermined variable potential relative tosaid potential of said electron source and for varying said surroundingpredetermined retarding potential over a predetermined potential range;

means for evacuating the region occupied by said collision space andsaid variable retarding potential to a predetermined pressure;

upon said electrons being introduced into said collision space, saidelectrons acquiring predetermined kinetic energy substantially equal inelectron volts to said relative predetermined potential in volts of saidcollision space whereby said gaseous material is impacted with saidelectrons of said predetermined kinetic energy;

said means for providing said collision space also for collectingelectrons having ins'ufficient residual kinetic energy after collidingwith said gaseous material to surmount the potential difference betweensaid predetermined potential of said collision space relative to saidelectron source and the varying potential of said variable surroundingretarding potential relative to said electron source upon the potentialdifference between said variable surrounding retarding potential andsaid potential of said electron source being at least less in volts thanthe kinetic energy lost by saidelectrons in electron volts upon saidelectrons colliding with said gaseous material, said collected electronsproviding a current; and

means for measuring said current as a function of said varyingsurrounding retarding potential.

45. Apparatus according to claim 44 wherein the potential of saidcollision space relative to said potential of said electron source is apredetermined relative positive potential and wherein the potential ofsaid surrounding variable potential relative to said potential of saidelectron source is varied between a relative potential more positivethan said relative positive potential of said collision space and arelative potential which is negative with respect to said potential ofsaid electron source.

46. Apparatus according to claim 44 wherein said means for mmeasuringsaid current as a function of said varying retarding potential comprisesmeans for taking the derivative of said current with respect to saidvary ing retarding potential.

47. Apparatus according to claim 44 further including means formodulating said variable retarding potential.

48. Apparatus according to claim 44 further including means forpre-analyzing the kinetic energy distribution of said electrons so as toimpact said gaseous material with a stream of electrons having apredetermined energy distribution about a predetermined energy means.

49. Apparatus according to claim 48 wherein said energy distribution isapproximately millivolts full width at half minimum.

50. Apparatus according to claim 44 further including means for varyingthe potential of said collision space relative to said potential of saidelectron source to vary the kinetic energy provided said electrons overa predetermined kinetic energy range.

51. Apparatus according to claim 50 wherein said means for varying thepotential of said collision space relative to said potential of saidelectron source is for varying such potential in a predetermined mannerso as to vary said kinetic energy provided said electrons in apredetermined manner with respect to said varying relative potential ofsaid surrounding retarding potential.

52. Apparatus according to claim 51 wherein said means for varying thepotential of said collision space relative to said potential of saidelectron source is for varying such potential proportionally withrespect to said varying potential of said surrounding retardingpotential with respect to said potential of said electron source so asto vary the kinetic energy provided said electrons proportionally withrespect to said varying potential of said surrounding retardingpotential with respect to said potential of said electron source.

53. Apparatus according to claim 51 wherein said means for carrying thepotential of said collision space relative to said potential of saidelectron source is for varying such potential so as to provide apredetermined constant potential difference between such potential andsaid varying potential of said surrounding retarding potential withrespect to said potential of said electron source so as to provide saidelectrons with varying kinetic energy in electron volts which is at apredetermined constant difference with respect to said varying potentialin volts of said surrounding retarding potential with respect to saidpotential of said electron source.

54. Apparatus for characterizing gaseous material, comprising:

means for providing a stream of electrons;

means for selecting a substream of said stream of electrons having anarrower energy spread than that of said stream of electrons;

grid means for establishing a region of positive potential, said regioncontaining gaseous material to be characterized, and said grid meanshaving a predetermined percentage of transmission for electrons;

electrode means surrounding said grid means, said electrode means forestablishing a retarding field region and for accepting an electroncurrent;

means for providing said electrode means with a varying potential;

means for introducing said substream of electrons into said region ofpositive potential to impact gaseous material received therein; and

means for measuring the electron current to said grid means as afunction of the potential of said electrode means, said currentcharacterizing said gaseous material.

55. Apparatus for characterizing gaseous material,

comprising:

means providing a source of electrons, each of said electrons from saidsource having approximately the same energy;

means providing a first region of space over which the potential energyof an electron relative to the reference zero of potential energy, whichreference zero of potential energy is defined as that potential energyfor electrons at which the average energy electron from said electronsource would possess zero kinetic energy, is substantially negative overa substantial portion thereof, said first region of space containingsaid gaseous material;

means providing a second region of space surrounding said first regionof space and over which second region of space the potential energy ofelectrons increases with separation from said first region of space;

means providing a surface at a predetermined variable potential relativeto said reference zero of potential and which surface bounds said secondre gion of space, upon said electrons from said source being introducedinto said first region of space and impacting said gaseous material,certain of said electrons passing through said surface and providing acurrent depending in magnitude upon said predetermined variablepotential, said dependence characterizing said gaseous material meansfor directing electrons from said source into said first region; andmeans for measuring said current as a function of said variablepotential.

1. The process of characterizing gaseous material, comprising the stepsof: surrounding said gaseous material in a region of predeterminedpotential with a predetermined electrostatic potential barrier;impacting said gaseous material with electrons of predetermined totalenergy; collecting electrons having a predetermined relationship betweentheir kinetic energy and the potential of said predetermined theirkinetic energy and the potential of said predetermined electrostaticpotential barrier to provide a current, said current characterizing saidgaseous material; and measuring said current.
 2. The process accordingto claim 1 wherein said collected electrons providing said current arerelated to the potential of said electrostatic potential barrier byhaving insufficient residual kinetic energy after colliding with saidgaseous material to surmount said electrostatic potential barrier. 3.The process according to claim 1 wherein said collected electronsproviding said current are related to the potential of saidelectrostatic potential barrier by having sufficient kinetic energy tosurmount said electrostatic potential barrier and wherein said currentincludes electrons having sufficient residual kinetic energy aftercolliding with said gaseous material to surmount said electrostaticpotential barrier.
 4. The process according to claim 1 including thefurther step of processing said current to characterize said gaseousmaterial.
 5. The process according to claim 4 including the further stepof modulating one of the potentials which establishes said predeterminedelectrostatic potential barrier and wherein said processing comprisesthe step of measuring said current as a function of said modulatedpotential.
 6. The process according to claim 5 wherein said processingcomprises the step of taking the derivative of said current with respectto said potential which is modulated.
 7. The process according to claim1 wherein said electrostatic potential barrier is varied over apredetermined range.
 8. The process according to claim 1 including thefurther step of modulating the energy of said electrons over apredetermined energy range.
 9. The process according to claim 1 whereinsaid electrostatic potential is provided by two potentials and whereinone of said potentials is varied in a predetermined manner with respectto the other.
 10. The process according to claim 9 wherein said onepotential is varied proportionally with respect to said other potential.11. The process according to claim 9 wherein said one potential isvaried to provide a constant predetermined potential difference betweensaid one potential and said other potential.
 12. The process accordingto claim 1 wherein said predetermined potential of said region is asubstantially uniform potential.
 13. The process according to claim 1wherein said predetermined potential of said region is a varyingpotential.
 14. The process according to claim 1 including the furtherstep of pre-analyzing the kinetic energy distribution of said electronsimpacting said gaseous material so as to impact said gaseous materialwith a stream of electrons having a predetermined energy distributionabout a predetermined energy average.
 15. The process according to claim14 wherein said energy distribution is approximately 100 millivolts fullwidth at half maximum.
 16. The process according to claim 1 includingthe further step of modulating said varying retarding potential.
 17. Theprocess according to claim 16 wherein said energy distribution isapproximately 100 millivolts full width at half maximum.
 18. The processof characterizing gaseous material, comprising the steps of: providing astream of electrons from an electron source at a predeterminedpotential; providing a collision space at a predetermined potentialrelative to said potential of said electron source, said predeterminedrelative potential being greater in volts than the kinetic energy lostby said electrons in electron volts upon said electrons colliding withsaid gaseous material; substantially surrounding said collision spacewith a variable retarding potential at a predetermined potentialrealtive to said potential of said electron source; evacuating theregion occupied by said collsion space and said variable retardingpotential to a predetermined pressure; introducing said gaseous materialinto said collision space; introducing said electrons into saidcollision space to provide said electrons with predetermined kineticenergy substantially equal in electron volts to said predeterminedpotential of said collision space and impacting said gaseous materialwith said electrons provided with said predetermined kinetic energy;varying the predetermined potential of said variable retarding potentialover a predetermined potential range; providing a current by collectingelectrons having insufficient residual kinetic energy after collidingwith said gaseous material to surmount the potential difference betweenthe potential of said collision space relative to said electron sourceand the varying potential of said variablE retarding potential relativeto said electron source upon the potential diffference between saidvariable retarding potential and said potential of said electron sourcebeing at least less in volts than the kinetic energy lost by saidelectrons in electron volts upon said electrons colliding with saidgaseous material; and measuring said current as a function of saidvarying retarding potential.
 19. The process according to claim 16including the further step of varying the potential of said collisionspace relative to said potential of said electron source to vary thekinetic energy provided said electrons over a predetermined kineticenergy range.
 20. The process according to claim 19 wherein thepotential of said collision space relative to said potential of saidelectron source is varied in a predetermined manner with respect to saidvarying potential of said surrounding retarding potential relative tosaid potential of said electron source so as to vary said kinetic energyprovided said electrons in a predetermined manner with respect to saidvarying relative potential of said surrounding retarding potential. 21.The process according to claim 20 wherein said potential of saidcollision space relative to said potential of said electron source isvaried proportionally with respect to said varying potential of saidsurrounding retarding potential with respect to said potential of saidelectron source so as to vary the kinetic energy provided said electronsproportionally with respect to said varying potential of saidsurrounding retarding potential with respect to said potential of saidelectron source.
 22. The process according to claim 20 wherein saidpotential of said collision space relative to said potential of saidelectron source is varied to provide a predetermined constant potentialdifference with respect to said varying potential of said surroundingretarding potential with respect to said potential of said electronsource so as to provide said electrons with varying kinetic energy inelectron volts which is at a predetermined constant difference withrespect to said varying potential of said surrounding retardingpotential with respect to said potential of said electron source involts.
 23. The process according to claim 22 wherein said predeterminedconstant potential difference is at least one volt.
 24. The processaccording to claim 22 wherein said predetermined constant potentialdifference is between 1 and 6 volts.
 25. The process according to claim18 wherein the potential of said collision space relative to saidpotential of said electron source is a predetermined relative positivepotential and wherein the potential of said surrounding variablepotential relative to said potential of said electron source is variedbetween a relative potential more positive than said relative positivepotential of said collision space and a relative potential which isnegative with respect to said potential of said electron source.
 26. Theprocess according to claim 18 wherein said step of measuring saidcurrent as a function of said varying retarding potential comprises thestep of taking the derivative of said current with respect to saidvarying retarding potential.
 27. The process according to claim 18including the further step of pre-analyzing the kinetic energydistribution of said electrons from said electron source to provide astream of electrons for impacting said gaseous material having apredetermined energy distribution about a predetermined average.
 28. Theprocess according to claim 27 including the further step of modulatingthe energy of said stream of electrons.
 29. The process ofcharacterizing gaseous material, comprising the steps of: providing asource of electrons, each of said electrons from said source havingapproximately the same energy; providing a first region of space overwhich the potential energy of an electron relative to the reference zeroof potential energy, which referEnce zero of potential energy is definedas that potential energy for electrons at which the average energyelectron from said electron source would possess zero kinetic energy, issubstantially negative over a substantial portion thereof, said firstregion of space containing said gaseous material; providing a secondregion of space surrounding said first region of space and over whichsecond region of space the potential energy of electrons increases withseparation from said first region of space; bounding said second regionof space with a surface at a predetermined variable potential relativeto said reference zero of potential, upon said electrons from saidsource being introduced into said first region of space and impactingsaid gaseous material, certain of said electrons passing through saidsurface and providing a current depending in magnitude upon saidpredetermined variable potential, said dependence characterizing saidgaseous material directing electrons from said source into said firstregion; and measuring said current as a function of said variablepotential.
 30. Apparatus for characterizing gaseous material,comprising: means for providing a stream of electrons; means forsurrounding said gaseous material with an electrostatic potential at apredetermined potential, for providing said electrons with predeterminedkinetic energy and for impacting said gaseous material with electronsprovided with predetermined kinetic energy; and means for measuringcurrent provided by electrons having a predetermined relationshipbetween their kinetic energy and said predetermined potential of saidelectrostatic potential, said current characterizing said gaseousmaterial.
 31. Apparatus according to claim 30 wherein said currentmeasuring means is for measuring current provided by electrons that arerelated to said predetermined potential of said electrostatic potentialby having insufficient residual kinetic energy after colliding with saidgaseous material to surmount said electrostatic potential.
 32. Apparatusaccording to claim 30 wherein said current measuring means is formeasuring current provided by electrons that are related to saidpredetermined potential of said electrostatic potential by havingsufficient kinetic energy to surmount said electrostatic potential andwherein said measured electrons include electrons having sufficientresidual kinetic energy after colliding with said gaseous material tosurmount said electrostatic potential.
 33. Apparatus according to claim30 wherein said current measuring means comprises means for measuringsaid current as a function of said predetermined electrostaticpotential.
 34. Apparatus according to claim 30 wherein said currentmeasuring means comprises means for taking the derivative of saidcurrent with respect to said predetermined electrostatic potential. 35.Apparatus according to claim 30 further including means for varying saidelectrostatic potential over a predetermined range.
 36. Apparatusaccording to claim 35 further including means for varying said kineticenergy provided said electrons over a predetermined kinetic energyrange.
 37. Apparatus according to claim 36 wherein said means formeasuring said electron current to said grid means measure said electroncurrent as a function of said varying potential of said electrode means,said function characterizing said gaseous material.
 38. Apparatusaccording to claim 36 wherein said means for providing said electronswith varying kinetic energy provides said electrons with kinetic energywhich is varied in a predetermined manner with respect to said varyingelectrostatic potential.
 39. Apparatus according to claim 38 whereinsaid means for varying said kinetic energy provided said electronsvaries said kinetic energy to provide a constant predetermined potentialdifference between said varying kinetic energy and said varyingelectrostatic potential.
 40. Apparatus according to claim 38 whereinsaid mEans for measuring said electron current as a function of saidvarying potential of said electrode means is for taking the derivativeof said electron current with respect to said varying potential of saidelectrode means.
 41. Apparatus according to claim 38 wherein said meansfor varying said kinetic energy provided said electrons varies saidkinetic energy proportionally with respect to said varying electrostaticpotential.
 42. Apparatus according to claim 41 wherein said derivativeis the second derivative of said electron current with respect to saidvarying electrode means potential.
 43. Apparatus according to claim 41wherein said derivative is the third derivative of said electron currentwith respect to said varying potential of said electrode means. 44.Apparatus for characterizing gaseous material, comprising the steps of:means for providing a stream of electrons from an electron source at apredetermined potential; means for providing a collision space at apredetermined substantially uniform, potential relative to saidpotential of said electron source, said predetermined relative potentialbeing greater in volts than the kinetic energy lost by said electrons inelectron volts upon said electrons colliding with said gaseous material,and said collision space containing said gaseous material; means forintroducing said electrons into said collision space to impact saidgaseous material; means for substantially surrounding said collisionspace with a variable retarding potential at a predetermined variablepotential relative to said potential of said electron source and forvarying said surrounding predetermined retarding potential over apredetermined potential range; means for evacuating the region occupiedby said collision space and said variable retarding potential to apredetermined pressure; upon said electrons being introduced into saidcollision space, said electrons acquiring predetermined kinetic energysubstantially equal in electron volts to said relative predeterminedpotential in volts of said collision space whereby said gaseous materialis impacted with said electrons of said predetermined kinetic energy;said means for providing said collision space also for collectingelectrons having insufficient residual kinetic energy after collidingwith said gaseous material to surmount the potential difference betweensaid predetermined potential of said collision space relative to saidelectron source and the varying potential of said variable surroundingretarding potential relative to said electron source upon the potentialdifference between said variable surrounding retarding potential andsaid potential of said electron source being at least less in volts thanthe kinetic energy lost by said electrons in electron volts upon saidelectrons colliding with said gaseous material, said collected electronsproviding a current; and means for measuring said current as a functionof said varying surrounding retarding potential.
 45. Apparatus accordingto claim 44 wherein the potential of said collision space relative tosaid potential of said electron source is a predetermined relativepositive potential and wherein the potential of said surroundingvariable potential relative to said potential of said electron source isvaried between a relative potential more positive than said relativepositive potential of said collision space and a relative potentialwhich is negative with respect to said potential of said electronsource.
 46. Apparatus according to claim 44 wherein said means formmeasuring said current as a function of said varying retardingpotential comprises means for taking the derivative of said current withrespect to said varying retarding potential.
 47. Apparatus according toclaim 44 further including means for modulating said variable retardingpotential.
 48. Apparatus according to claim 44 further including meansfor pre-analyzing the kinetic energy distribution of said electronS soas to impact said gaseous material with a stream of electrons having apredetermined energy distribution about a predetermined energy means.49. Apparatus according to claim 48 wherein said energy distribution isapproximately 100 millivolts full width at half minimum.
 50. Apparatusaccording to claim 44 further including means for varying the potentialof said collision space relative to said potential of said electronsource to vary the kinetic energy provided said electrons over apredetermined kinetic energy range.
 51. Apparatus according to claim 50wherein said means for varying the potential of said collision spacerelative to said potential of said electron source is for varying suchpotential in a predetermined manner so as to vary said kinetic energyprovided said electrons in a predetermined manner with respect to saidvarying relative potential of said surrounding retarding potential. 52.Apparatus according to claim 51 wherein said means for varying thepotential of said collision space relative to said potential of saidelectron source is for varying such potential proportionally withrespect to said varying potential of said surrounding retardingpotential with respect to said potential of said electron source so asto vary the kinetic energy provided said electrons proportionally withrespect to said varying potential of said surrounding retardingpotential with respect to said potential of said electron source. 53.Apparatus according to claim 51 wherein said means for carrying thepotential of said collision space relative to said potential of saidelectron source is for varying such potential so as to provide apredetermined constant potential difference between such potential andsaid varying potential of said surrounding retarding potential withrespect to said potential of said electron source so as to provide saidelectrons with varying kinetic energy in electron volts which is at apredetermined constant difference with respect to said varying potentialin volts of said surrounding retarding potential with respect to saidpotential of said electron source.
 54. Apparatus for characterizinggaseous material, comprising: means for providing a stream of electrons;means for selecting a substream of said stream of electrons having anarrower energy spread than that of said stream of electrons; grid meansfor establishing a region of positive potential, said region containinggaseous material to be characterized, and said grid means having apredetermined percentage of transmission for electrons; electrode meanssurrounding said grid means, said electrode means for establishing aretarding field region and for accepting an electron current; means forproviding said electrode means with a varying potential; means forintroducing said substream of electrons into said region of positivepotential to impact gaseous material received therein; and means formeasuring the electron current to said grid means as a function of thepotential of said electrode means, said current characterizing saidgaseous material.
 55. Apparatus for characterizing gaseous material,comprising: means providing a source of electrons, each of saidelectrons from said source having approximately the same energy; meansproviding a first region of space over which the potential energy of anelectron relative to the reference zero of potential energy, whichreference zero of potential energy is defined as that potential energyfor electrons at which the average energy electron from said electronsource would possess zero kinetic energy, is substantially negative overa substantial portion thereof, said first region of space containingsaid gaseous material; means providing a second region of spacesurrounding said first region of space and over which second region ofspace the potential energy of electrons increases with separation fromsaid first region of space; means providing a surface at a predetermiNedvariable potential relative to said reference zero of potential andwhich surface bounds said second region of space, upon said electronsfrom said source being introduced into said first region of space andimpacting said gaseous material, certain of said electrons passingthrough said surface and providing a current depending in magnitude uponsaid predetermined variable potential, said dependence characterizingsaid gaseous material means for directing electrons from said sourceinto said first region; and means for measuring said current as afunction of said variable potential.